1. Field of the Invention
The present invention relates generally to a method and apparatus for modulating the amplitude or intensity of a light beam, and more particularly to an electro-optic attenuated total internal reflection modulator and method for amplitude modulating light from narrow-band light sources like lasers and light-emitting diodes (LEDs). The bandwidth of the modulation ranges from DC to microwave frequencies (beyond 100 GHz), and the concept is applicable to all narrow-band sources in the optical spectrum. The modulation can be analog or digital, small signal or large signal depending on the application.
2. Brief Description of the Prior Art
Heretofore, electro-optic materials have been used in various types of light-modulating apparatus. See, for example, "High-Speed Optical Modulator for Application in Instrumentation" by R. L. Jungerman et al., Journal of Lightwave Technology, Volume VIII, No. 9, pages 1363-1370 (Sep. 1990); "Surface Plasmon Spatial Light Modulators", by E. M. Yeatman et al., S.P.I.E., Volume 1151, pages 522-532, Optical Information Processing Systems and Architectures (1989); "Surface Plasmon Enhanced Intersubband Resonance in AlGaAs QWs" by M. J. Cain et al., S.P.I.E., Volume 861, pages 82-85, Quantum Wells and Super Lattices in Opto-Electronic Devices and Integrated Optics (1987); U.S. Patent to R. T. Collins et al. U.S. Pat. No. 4,915,482 entitled "Optical Modulator".
Moreover, organic electro-optic (E-O) materials have been the subject of increasing study for over fifteen years. Such organic materials can exhibit extremely large broad-band electro-optical response while maintaining a low dielectric constant from DC through the visible range. These materials may currently be classified into four subgroups: crystalline, cross-linked polymers, pendant side-chain polymers, and guest-host systems. In all four subgroups the origin of the E-O response lies in the asymmetric electronic structure of the nonlinear optic (NLO) molecular units. In order for the bulk of the material to exhibit an electro-optic response, the NLO units must comprise a noncentrosymmetric structure.
Some NLO molecules form naturally into noncentrosymmetric crystals and make up the first subgroup. The remaining three subgroups also employ NLO molecular units but, while still having a noncentrosymmetric structure, do not possess any crystalline order. They differ from each other in the number of covalent bonds per NLO unit that exist between the unit and either an optically inactive support material or other NLO units. Cross-linked polymers, pendant side-chain polymers and guest-host materials possess two, one and no covalent bonds to the NLO units respectively. Unlike the crystalline materials, these three types of organic E-O materials do not usually form naturally into noncentrosymmetric structures. After forming them into the desired geometry, an additional processing step is required to partially orient the NLO units and create a bulk noncentrosymmetric structure. However, the relative ease in which the noncrystalline materials can be fabricated into device architectures more than makes up for the additional processing step required.
The orientation of the NLO units is referred to as poling. A poling operation usually involves heating the organic layer to a temperature near or above its glass transition temperature where the NLO units become partially mobile. A strong electric field is then applied either by electrodes adjacent to the layer or by using a corona discharge to deposit electrons on one side of the organic layer. The electric field exerts a torque on the NLO units which by virtue of their asymmetric electronic structure possess large dipole moments. After the NLO units have partially aligned with the electric field as a result of the induced torque, the temperature is reduced back to ambient with the field still applied to freeze in the partial alignment. The resulting material then has a noncentrosymmetric structure and exhibits a linear E-O response. Noncrystalline organic E-O materials can be processed at temperatures compatible with many integrated circuit technologies and can be formed into many quality thin films making them particularly useful in integrated optic and optical modulator applications.
The magnitude of the electro-optic effect is a function of the wavelength of the light being modulated. This dependence on wavelength is strongest when the wavelength is close to the excitation energy of the NLO units. Experimental data on 2-methyl-4-nitroaniline (MNA) indicates that at least 5-fold increases in the electro-optic response can be achieved by operating near the excitation wavelength. The drawback to operating at wavelengths near the excitation wavelength is that the optical absorption of the organic material also rises rapidly in this wavelength region. In most modulator geometries, a large increase in the optical absorption of the active layer results in an unacceptable reduction in the amount of light that the modulator transmits.
However, it has been found that, if a particular thickness of poled organic film of the type described above is interfaced to a metal layer to form a light reflecting device, the photons of an incoming light beam intercepting the device at a specific angle will be phase-matched to surface plasmons at the film-to-metal interface, and the resonant coupling of energy from the light beam will result in a sharp minimum of reflectance. But since the phase-match angle is dependant on the relative dielectric constant of the organic material, and such constant can be changed by the application of an electric field across the film, it follows that one can influence the reflectance of, and then modulate the intensity of the reflected beam by changing the applied electric field. This modulator geometry can operate effectively even if the organic film has high optical absorption (&gt;200 cm.sup.-1).